drm/i915/lt_phy: Add xe3plpd Thunderbolt PLL hooks

drm/i915/lt_phy: Add xe3plpd Thunderbolt PLL hooks

Add the PLL hooks for the TBT PLL on xe3plpd. These are simple stubs
similar to the TBT PLL on earlier platforms, since this PLL is always
on from the display POV - so no PLL enable/disable programming is
required as opposed to the non-TBT PLLs - and the clocks for different
link rates are enabled/disabled at a different level, via the
intel_encoder::enable_clock()/disable_clock() interface.

Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://patch.msgid.link/20260312080657.2648265-22-mika.kahola@intel.com